The present invention relates to a new and improved construction of a fire alarm having a radiation source operated in a pulsed mode, the radiation source transmitting electromagnetic radiation to a measuring chamber which is accessible to air to be examined for the presence of smoke and aerosol particles.
Such fire alarms known also as optical smoke detectors, exploit the fact that the radiation, for instance, ultraviolet, visible light or infrared radiation, transmitted by a radiation source in a measuring chamber, is affected in a certain fashion upon presence of smoke particles or aerosols stemming from a combustion process in the measuring chamber.
Preferably, these fire alarms operate according to the principle of scattered radiation. There is provided a scattered radiation receiver which is not directly impinged by direct radiation, this receiver receiving radiation scattered by the smoke particles or the like and triggering a fire alarm signal as soon as the scattered radiation intensity exceeds a predetermined threshold. However, what is disadvantageous with such fire alarms is the fact that, they only respond to smoke which has a pronounced scattering effect upon the radiation, so-called white smoke, as for instance is formed during the combustion of moist or wet materials. They do not however respond to intensely radiation-absorbing smoke, and thus, in other words to smoke which only produces very little scattered radiation, so-called black smoke, as the same frequently is produced in the case of rapidly progressing fires or in the event of incomplete combustion processes. Heretofore known scattered radiation fire alarms are therefore not capable of reporting combustion processes of the type which are associated with the occurrence of markedly radiation-absorbent smoke, in other words black smoke. This is especially disadvantageous in the case of rapidly progressing fires, where the scattered radiation fire alarm frequently first triggers an alarm signal much too late in time.
Other heretofore known optical smoke detectors operate in accordance with the extinction principle. Here, a radiation receiver is directly irradiated by the radiation source. Upon the presence of smoke the irradiation thereof diminishes owing to the radiation absorption which occurs at the smoke particles and the radiation scattering. With a certain reduction in the irradiation of the radiation receiver there is triggered an alarm signal. Such fire alarms are capable of detecting even strongly absorbing, in other words black smoke, but however they require relatively large absorption paths, in the order of magnitude of one meter, if, as is necessary in practical applications, there should be detected with adequate sensitivity even small smoke densities. Therefore, such fire alarms are difficult to fabricate in the dimensions of at most ten centimeters which are needed in practical applications, if there are not to be used complicated, sensitive, expensive and dust-prone deflection mirror systems.
Extinction fire alarms are capable of detecting different types of smoke with relatively uniform sensitivity. However, they are associated with the limitation that a relatively small change of a relatively large irradiation value must be positively detected. In practical terms, this means that there is required an extremely good and correspondingly complicated and expensive long time stabilization of the radiation source. Therefore, in practice scattered light-fire alarms have found extensive acceptance in those situations where there is to be determined a deviation of a magnitude from null, something which can be accomplished much easier and without any great expenditure in equipment for stabilizing the radiation source. But, with this technique there must still be tolerated the drawback that such scattered lightfire alarms do not respond to all types of fires or combustion processes.
A further disadvantage which is associated with all heretofore known optical fire alarms resides in the fact that they only respond to smoke particles whose dimensions are greater than approximately those of the radiation wavelengths, i.e., greater than about 1.mu.. Smaller particles, which tend to form at the incipient stage of a fire, cannot be detected, so that such optical fire alarms frequently first then trip an alarm signal much too late in time. Consequently, it is therefore necessary to prefer other more rapidly responsive types of fire alarms, such as, for instance, ionization fire alarms. But ionization fire alarms are also afflicted with the shortcoming that it is necessary to use radioactive substances, which, in turn, again have other undesirable affects.